00:01
All right, so let's say we have a ballistic pendulum with a block hanging from a string, and the block has a mass, we use a capital in for this, of 0 .86 kilograms, and a bullet is fired into the block at a speed of 205 meters per second, and the bullet has a mass of 18 grams, so 0 .18 kilograms.
00:27
And it embeds itself in the block and the block swings back and forth.
00:32
And part a asks us, determine the speed of the block plus bullet system immediately after the collision.
00:37
So let's call this vb, like speed of the bullet.
00:42
So the initial momentum is just going to be m times vb.
00:45
And the final momentum is going to be m plus m because the bullet is embedded in the system times their shared velocity.
00:52
So that's what we want to find.
00:53
We want to find v.
00:54
So we can see it's the moment.
00:56
Momentum of the bullet divided by the combined masses of the two.
01:02
And so we don't even need to write the masses in kilograms.
01:07
If you don't want to, you can just write this as 18 over 878 times 205 meters per second because the units are going to cancel out.
01:22
And so this gives us a speed of about 4 .2 meters...